1. Field of the Invention
This invention relates to photographic lenses utilizing "floating" and which are suited to cameras for photography, video cameras, etc. and, more particularly, to high performance photographic lenses utilizing floating and well corrected for aberrations when focusing on an object in a wide range from an infinitely distant object to a close object for high image magnification.
2. Description of the Related Art
In the past, in still cameras and video cameras, among the photographic lenses having their main aim to shoot close objects, there is what is called the macro lens or the micro lens (hereinafter referred to as "macro lens".) The macro lens is designed so as to obtain high optical performance particularly on close objects compared with the other general photographic lenses such as standard lenses, telephoto lenses, etc. Also, the macro lens in many cases is used even on objects of a wide range from a close object to an infinitely distant object.
In general, in the macro lens, when the range of image magnifications is extended, particularly when extended toward higher magnifications, a change of image magnification is accompanied by production of many variations in aberration, and it becomes difficult to correct this.
For example, in a macro lens having a large aperture ratio, when focusing is effected down to a close-up region, the spherical aberration becomes under-corrected, and the curvature of field is also worsened, and further, a large outward coma is produced. Particularly, the astigmatism and coma are increased largely in terms of 3rd and 5th order ones. Good correction of these aberrations becomes very difficult to maintain in a stable condition.
The method for correcting the variation of aberrations of a large relative aperture lens such as the GAUSS lens with variations of the image magnification when shooting from an infinitely distant object to a close object is known, for example, as discussed in U.S. Pat. No. 2,537,912, and U.S. Pat. No. 3,815,974. Also, recent proposals are made in Japanese Laid-Open Patent Applications Nos. Sho 48-90520, 52-7723, 57-8513, 57-46222, 57-192916 (U.S. Pat. No. 4,553,823), 59-143116, 62-177509 (U.S. patent application Ser. No. 007,626) and 62-177510. Any of the photographic lenses proposed in these documents utilizes at least two lens units made to move independently when focusing (so-called "floating"). However, any of the photographic lenses proposed in these documents corrects for aberration in near distance shooting of low magnification, but the correction effect is not always sufficient in shooting high magnification object, for example, life-size or thereabout. For example, the coma is relatively well corrected in low magnification, but as the magnification becomes high, there is a tendency to produce large distortion.